At the end, this article says ₹1,500 crore for 0.35 MT greenfield, with NINL being ~3MT, greenfield cost would be atmost ₹15,000 crores. At higher scale like 3MT, cost could be significantly lower. Not sure if NINL has integrated power plant, so by and large based on tech, age, maintenance etc., should be in ₹15,000 crores ballpark or less.
So ques becomes is NMDC worth ₹20,000 crores now (35-15) incl cash.